The state-of-health of a lithium ion battery measures the fraction of life that remains should the battery continue to be operated in the same way, for example for a battery in a portable phone or a battery in an electric vehicle. The fraction of life remaining can be defined by the ratio of the number of remaining charge-discharge cycles (to occur in the future) to the total number of charge-discharge cycles that will occur over the entire life of the battery. Alternatively, the fraction of life remaining can be defined by the ratio of the remaining time (to occur in the future) that the battery will be useful to the total time that that battery will be useful. The state-of-health of any rechargeable or secondary electrochemical device can defined in a similar way.
Measuring or estimating the state-of-health of an arbitrary Li-ion battery is very difficult to do. One may not know the age of the battery, the number of charge-discharge cycles it has undergone, or the initial characteristics of the cells within the battery. Nevertheless, there are proposals to take Li-ion batteries from used electric vehicles and re-use them for grid energy storage. In such an enterprise, it will be important to know if some batteries are expected to have, e.g. 10% life remaining while others have, e.g. 90% remaining, so that when these are connected in an energy storage facility the batteries can be connected appropriately to ensure ease of replacement at an appropriate time. In addition, batteries in electric vehicles age at different rates depending on numerous factors including temperature history, driving habits, charging potential, chemistry of the Li-ion cells selected, etc., so it is important to know their state-of-health before re-using or replacing them.
Most methods to estimate state-of-health of Li-ion batteries rely upon the observation that the internal resistance or impedance of Li-ion cells generally increases with battery age. During storage and during charge-discharge cycling, reactions between the electrode materials and the electrolyte occur which generally leads to the deposition of layers of reaction products on the electrode particle surfaces and which thus increase the battery cell impedance. Therefore, if one has previous knowledge of the maximum value of the internal impedance that renders the battery still useful in the intended application, then a comparison of the present impedance to that maximum useful impedance yields a reasonable predictor of the state-of-health.
As examples of related prior art, U.S. Pat. No. 8,415,926 discloses impedance measurements to estimate state-of-health. U.S. Pat. No. 8,937,459 discloses comparisons of voltage and current to a look-up table on batteries of known degree of degradation to estimate battery state-of-health. U.S. Pat. No. 8,427,166 discloses electrical measurements of battery capacity and voltage to determine state-of-health. U.S. Pat. No. 8,589,097 discloses a method that compares the voltage of a battery under load and after elimination of load (open circuit) to estimate state-of-health. U.S. Pat. No. 8,680,815 discloses a method involving comparisons of dQ/dV (differential capacity) vs V (voltage) of the battery under load to those of a reference anode and cathode. U.S. Pat. No. 8,116,998 discloses a method in which internal resistances of batteries are compared to a predetermined critical resistance threshold. U.S. Pat. No. 7,554,294 discloses a method in which a full AC impedance spectrum is used to characterize battery health. U.S. Pat. No. 6,456,043 discloses a method of monitoring of voltage and capacity during storage periods to determine state-of-health of the battery. All the aforementioned prior art references use some sort of electrical measurement to determine the state-of-health and require prior knowledge of the battery behaviour throughout its lifetime.
By contrast, US Patent Application 2014/0107949 discloses a method in which a stress/strain sensor mounted on the battery is used to determine the state-of-health of a battery by comparison to previously measured stress/strain data for batteries at a different state of charge and different state-of-health. This method involves substantial prior knowledge of the battery behaviour throughout its entire lifetime.
Recent publications on next generation high-voltage Li-ion cells have shown that appropriate electrolyte additives can mitigate against steady impedance increase but that Li-ion cells still show failure, instead, by rapid impedance growth very near end of life. [For example, K. J. Nelson, G. L. d'Eon, A. T. B. Wright, L. Ma, J. Xia and J. R. Dahn, Studies of the effect of high voltage on the impedance and cycling performance of Li[Ni0.4Mn0.4Mn0.2]O2/graphite lithium-ion pouch cells, J. Electrochem. Soc. 2015 162(6): A1046-A1054; doi: 10.1149/2.0831506jes and Mengyun Nie, Jian Xia and J R. Dahn, Development of Pyridine-Boron Trifluoride Electrolyte Additives for Lithium-Ion Batteries. J. Electrochem. Soc. 2015 162(7): A1186-A1195; doi: 10.1149/2.0271507jes]. This means that traditional state-of-health measurements may not yield accurate assessments of the fraction of life remaining. A new method for state-of-health determination is required.
The present invention addressed these needs and provides further benefits as disclosed below.